Melt containment apparatus for containing molten metals are made from various materials which are selected for their anti-corrosive properties. Molten metals such as steel, gray iron and aluminum pose the greatest corrosion problems during handling. Zinc and copper are much easier to handle, so suitable materials for handling iron and aluminum will generally be able to handle molten zinc and copper. For instance, ladles required for the holding and processing of molten aluminum are typically made of a refractory material (carbon-bonded silicon carbide or clay-graphite) or refractory-coated cast iron. However, refractory materials are relatively weak, requiring large wall thicknesses to provide adequate strength. This results in a relatively high initial cost. On the other hand, refractory-coated cast iron crucibles are stronger and cheaper, but must be recoated frequently because the underlying cast iron is usually attacked by the molten aluminum being held. Coated steel ladles are generally not used because steel is even more rapidly attacked by molten aluminum than cast iron.
In addition, when casting molten iron or other ferrous materials in permanent molds, a number of problems result due to the higher molten metal temperatures required to obtain satisfactory components (approximately 650.degree. C. is needed for aluminum while at least 1300.degree. C. is required for iron and up to 1700.degree. C. for steel). The molds used to cast the molten iron and other ferrous materials experience poor die life due to thermal fatigue cracking and molten metal attack. Furthermore, the difficulties of maintaining high die temperatures to avoid premature freezing of the molten iron in the mold have limited the commercial development of ferrous permanent mold casting and have also discouraged attempts at commercializing die casting of iron and steel. Typically, methods other than mold casting are used to form cast iron parts, such as sand casting.
The most widely used material for permanent molds for handling such ferrous material castings has been gray iron. Gray iron or alloy gray iron may be used as a mold material for both low and high volume production of zinc, aluminum and copper castings. In the past, poor service life of gray iron molds has been due to dimensional instability and has consequently restricted permanent mold casting of gray iron parts to low volume production. For die casting of zinc, aluminum and copper, chromium-tungsten hot work die steels and tungsten hot work die steels are typically used as mold materials. Currently, iron and steel are generally not die cast.
The primary limitations of gray iron, alloy gray iron, and the hot work die steel molds used for casting iron and steel have been molten metal attack, property degradation and dimensional instability. These limitations arise from the fact that when the mold is kept at a sufficiently high temperature to prevent premature freezing, they also cause iron carbide dissolution, scaling (oxidation), alpha-iron to gamma-iron transformation, and .RTM.v.RTM.n melting. Scaling produces changes in the interior mold dimensions because the scale must be removed by abrasive cleaning. Iron carbide dissolution or alpha-to-gamma transformation results in changes in both mold dimensions and mechanical properties.
Gray iron may be cast in gray iron permanent molds if the mold equilibrium temperature is kept below the alpha-to-gamma transformation temperature because the hot cast metal and molds are cooled gradually. However, in die casting, molten metal is injected into the mold and the metal and mold are cooled as quickly as possible. Due to the quick cooling, gray iron or steel cannot be die cast into gray iron or steel molds because the cooling required to maintain a mold equilibrium temperature below the alpha-to-gamma transformation temperature produces sufficiently large thermal gradients to cause cracking. Numerous attempts at developing mold materials (primarily refractory metal alloys such as molybdenum alloys) for die casting iron and steel have been unsuccessful in demonstrating sufficiently long die life to be economically feasible.
Coatings such as soot and refractory oxides are often employed in the mold to prevent the atmosphere and molten metal from contacting the die material. Although these coatings may inhibit chemical attack, they are disadvantageous because they also significantly alter heat transfer. Furthermore, these coatings are not permanent and must be replaced frequently, thus increasing down time and cost. Mold materials which do not rely on sacrificial coatings for protection are also desirable because some processes, such as vacuum and pressure casting, produce high velocity molten metal which can cause significant erosion. 5 Therefore, it is understood that there is a need in the foundry industry for an improved material which can be used to contain molten metals, such as aluminum and iron. The various melt containment apparatuses include ladles, furnaces, molds and any other article which contains molten metal for processing. Considering this need, it is a primary object of the present invention to provide a material in accordance with the present invention which is suitable for making containment vessels for molten metals, especially a material for containing molten aluminum and iron which does not dissolve or spall to contaminate the molten metal.
It is another object of the present invention to provide a material which: (1) is resistant to attack from molten metal, property degradation and dimensional instability, (2) has high temperature strength, (3) is highly fabricable (since machining costs often greatly exceed material costs), and (4) is economical and long-lasting to eliminate the down time needed to replace sacrificial mold coatings. Further, it is yet another object of the present invention to provide a material which will make die casting of molten iron commercially viable.